Magnetic resonance laboratory experiments

The CH radical is the simplest hydrocarbon and its rotational or /I-doublet spectrum has been sought by many. The first detection of rotational transitions was a triumph for far-infrared laser magnetic resonance the experiments carried out by Evenson, Radford and Moran [173] were described in detail in chapter 9. The A-doublet transition in the lowest rotational level was first observed through radioastronomy by Rydbeck, Ellder, Irvine, Sume and Hjalmarson [174]. It was almost a further ten years before laboratory observations of the field-free spectrum were reported. 

Due to the high information content of nuclear magnetic resonance (NMR) experiments the application of NMR-based process control systems becomes increasingly attractive. Especially low field NMR systems enhance the applicability of NMR based methods in laboratory and industry, being small, mobile, maintenance fnendly, inexpensive and easier adaptable to their surroundings. 

The spectroscopic techniques that have been most frequently used to investigate biomolecular dynamics are those that are commonly available in laboratories, such as nuclear magnetic resonance (NMR), fluorescence, and Mossbauer spectroscopy. In a later chapter the use of NMR, a powerful probe of local motions in macromolecules, is described. Here we examine scattering of X-ray and neutron radiation. Neutrons and X-rays share the property of being found in expensive sources not commonly available in the laboratory. Neutrons are produced by a nuclear reactor or spallation source. X-ray experiments are routinely performed using intense synclirotron radiation, although in favorable cases laboratory sources may also be used. 

The use of computer simulations to study internal motions and thermodynamic properties is receiving increased attention. One important use of the method is to provide a more fundamental understanding of the molecular information contained in various kinds of experiments on these complex systems. In the first part of this paper we review recent work in our laboratory concerned with the use of computer simulations for the interpretation of experimental probes of molecular structure and dynamics of proteins and nucleic acids. The interplay between computer simulations and three experimental techniques is emphasized (1) nuclear magnetic resonance relaxation spectroscopy, (2) refinement of macro-molecular x-ray structures, and (3) vibrational spectroscopy. The treatment of solvent effects in biopolymer simulations is a difficult problem. It is not possible to study systematically the effect of solvent conditions, e.g. added salt concentration, on biopolymer properties by means of simulations alone. In the last part of the paper we review a more analytical approach we have developed to study polyelectrolyte properties of solvated biopolymers. The results are compared with computer simulations. 

Hyphenated analytical techniques such as LC-MS, which combines liquid chromatography and mass spectrometry, are well-developed laboratory tools that are widely used in the pharmaceutical industry. Eor some compounds, mass spectrometry alone is insufficient for complete structural elucidation of unknown compounds nuclear magnetic resonance spectroscopy (NMR) can help elucidate the structure of these compounds (see Chapter 20). Traditionally, NMR experiments are performed on more or less pure samples, in which the signals of a single component dominate. Therefore, the structural analysis of individual components of complex mixtures is normally time-consuming and less cost-effective. The... 

This laboratory manual assumes that the student is already familiar with organic chemistry and has taken a course in polymer chemistry where the mechanisms of the various polymer reactions illustrated by the preparations in this manual have already been covered. Careful record keeping is essential and is covered in a separate section later. Experience in the various analytical techniques such as infrared (IR) and nuclear magnetic resonance (NMR) is also assumed. Experience in distillation, both at atmospheric pressure and at reduced pressure, is also assumed. Where possible, monomers are used with little purification except for inhibitor removal and drying by students in order to save time. However, when careful kinetics are required, then very careful purification is a necessity. 

Clearly the development of experiments involving modem instrumentation is a vibrant part of Physical Chemistry laboratory pedagogy. Equally clearly, there is now a need to focus more strongly on experiments involving instruments other than lasers or magnetic resonance spectrometers. 

The spin-spin relaxation time can, in principle, be measured from the FID following the 90° pulse (one-pulse experiment Fig. 10a). However, the application of this simple experiment is limited only to very short FIDs, and consequently short T2s, because of the inhomogenities in the laboratory field. The line width in magnetic resonance is proportional to T2, but the observed linewidth,- T, has also a contribution from the magnetic field term ... 

Since the laser magnetic resonance experiment relies on a chance near-coincidence between a laser line and a molecular transition frequency, and the range over which spectroscopic transitions can be magnetically tuned is often quite small, it is desirable to have a large number of FIR laser lines available. This is now seldom a major problem, and table 9.1 lists a restricted sample of FIR laser lines that have been used for magnetic resonance studies. It is, of course, necessary to be able to measure the FIR frequency accurately and this is accomplished in Evenson s laboratory by measuring the beat... 

We have already discussed the high-resolution spectroscopy of the OH radical at some length. It occupies a special place in the history of the subject, being the first short-lived free radical to be detected and studied in the laboratory by microwave spectroscopy. The details of the experiment by Dousmanis, Sanders and Townes [4] were described in section 10.1. It was also the first interstellar molecule to be detected by radio-astronomy. In chapter 8 we described the molecular beam electric resonance studies of yl-doubling transitions in the lowest rotational levels, and in chapter 9 we gave a comprehensive discussion of the microwave and far-infrared magnetic resonance spectra of OH. Our quantitative analysis of the magnetic resonance spectra made use of the results of pure field-free microwave studies of the rotational transitions, which we now describe. 

The experience at the Radiation Lab was very important for Purcell s professional development. Thrown together with more senior physicists, he learned about their pre-Rad Lab work in physics. Particularly influential, as Purcell acknowledged, were the physicists from Rabi s laboratory at Columbia. Purcell learned from Rabi (to whom he reported), Ramsey, Zacharias, and Henry Torrey about Rabi s prewar magnetic resonance experiments, and as the war wore on, ideas began to take shape in Purcell s thinking. 

Another excellent method of quantitative analysis makes use of tagging with radioactive or stable isotopes [29,30]. Detection is by radiation, mass spectroscopy [31], or nuclear magnetic resonance [32], Unfortunately, experiments involving radioactivity require elaborate precautions and special laboratory rooms to avoid contamination and meet legal requirements, and mass-spectroscopic and NMR equipment is expensive and may not be available. 

Chemically induced dynamic nuclear polarization (CIDNP) is a nuclear magnetic resonance method based on the observation of transient signals, typically substantially enhanced, in either absorption of emission. These effects are induced as a result of magnetic interactions in radical or radical ion pairs on the nanosecond time scale. This method requires acquisition of an NMR spectrum during (or within a few seconds of) the generation of the radical ion pairs. The CIDNP technique is applied in solution, typically at room temperature, and lends itself to modest time resolution. The first CIDNP effects were reported in 1967, and their potential as a mechanistic tool for radical pair reactions was soon recognized [117, 118]. Nuclear spin polarization effects were discovered in reactions of neutral radicals and experiments in the author s laboratory established that similar eflects could also be induced in radical ions [119-121]. 

Even after I gave up doing experiments in my own laboratory, when I was about 45, I continued as a consultant during the summers in the Los Alamos National Laboratory. Some of my work on the interaction of solvent water with metal ions was done at Los Alamos, using nuclear magnetic resonance techniques. I believe ours was something of a pioneering effort in that field. 

Because use of mass spectrometry by chemists has increased greatly, most U.S. chemists have access to mass spectrometry facilities at their own institutions to confirm synthesis and support structure elucidations. Heavily used national centers provide more expensive instrumentation and more complex experiments. Most notably, a section of the National High Magnetic Field Laboratory at Florida State University provides state-of-the-art Fourier transform ion cyclotron resonance mass spectrometry. The NSF Arizona Accelerator Mass Spectrometry Laboratory is used primarily to provide radiocarbon measurements. NIH funds a number of national mass spectrometry centers to support biomedical research, including those at Boston University and the Pacific Northwest National Laboratory. 

Nuclear magnetic resonance spectroscopy provides the most conclusive evidence of both identity and purity [90] but few laboratories are equipped with such a resource and even fewer researchers with the experience to interpret the resulting data. Gas chromatography can be used to assess the chiral purity of derivatives, and mass spectrometry (MS) is a particularly sensitive and accurate measure of product purity. Use of electrospray ioni-... 

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